1. Field of the Invention
The invention relates in general to superabrasive compacts, and, in particular, to compatible ceramic filled porous high temperature monolithic materials, such as, for example, self-bonded polycrystalline diamond or cubic boron nitride, which have improved fracture resistance and mechanical strength. The pores of a self-bonded preform are lined with multiple layers of ceramic deposited throughout the preform by successive cycles of liquid pre-ceramic impregnation-cure-pyrolysis. The layers may or may not be distinct from one another in the finished superabrasive compact. The layers may or may not have the same properties.
2. Description of the Prior Art
It is well recognized that sintered diamond and cubic boron nitride compacts, wherein the self-bonded particles are formed into a porous monolith, are superabrasives. Typically, the self-bonded compacts have a network of connected pores that extend throughout the compact. As formed this network of pores is typically filed with some material, such as cobalt, that was used to aid in the formation of the self-bonded compact. Removal of this material leaves a compact with an open network of pores extending generally throughout the compact and the superabrasive particles bonded to one another. The empty porosity reduces the strength and the density, but increases the thermal stability from about 700 to 1200 degrees centigrade. Numerous proposals had been made to apply one or more ceramic or metal coatings to diamond particles and consolidate these into abrasive compacts. See, for example, Boyce U.S. Pat. No. 6,138,779, Ritt et al. U.S. Pat. No. 6,238,280, and Chen et al. U.S. Pat. No. 5,024,680. In these proposed abrasive compacts the superabrasive particles were not self-bonded. The superabrasive particles were bonded to one another through some other matrix material.
Currently, diamond and cubic boron nitride are the only known superabrasive materials. Such superabrasive compacts are highly desired for their ability to cut or abrade very hard materials. Superabrasive compacts find application in the mining and drilling industries where hard rock is being cut, as well as in the machining industries. See, for example, Boyce U.S. Pat. No. 6,138,779. It is also generally recognized that it would be very desirable to have superabrasive compacts that exhibit a combination of properties such as high heat resistance, high fracture resistance (toughness), and low wear rates. Numerous generally unsuccessful attempts had previously been made to achieve such a combination of properties. Typically, such previous attempts resulted in achieving to some limited degree one or two of these properties at the expense of the others. Fracture resistance is an important parameter for the performance of diamond cutters when drilling, for example, hard rock.
Durability is generally considered to be the ability of a cutting tool to retain its original sharpness. As a cutting tool becomes dull the rate at which it cuts decreases substantially. For rock drilling applications at substantial depths the cutting rate is very important, often more so than the length of the life of the tool. The durability of monolithic diamond compacts composed of direct bonded diamond particles with cobalt inclusions (PCD), formed under high pressures and temperatures, for example, from diamond grit and a cobalt sintering aid-binder, exhibit low heat resistance, which adversely affects the durability. The cobalt has a much higher coefficient of thermal expansion than the diamond and dissolves the diamond at elevated temperatures above approximately 700 degrees centigrade. Because of the mismatched thermal expansion rates, cobalt containing polycrystalline diamond compacts tend to microcrack, and otherwise loose structural integrity, at the elevated temperatures that are frequently encountered in drilling hard rock at commercially acceptable rates. When the cobalt is leached out, the resulting reticulated porous compact, sometimes described as a thermally stable polycrystalline diamond (TSP) compact, exhibits high heat resistance but low fracture resistance. The low fracture resistance adversely affects the durability of the compact.
The use, for example, of silicon carbide as the sintering binder in a diamond compact results in a compact with generally low fracture strength and wear resistance because of the limited direct binding of the diamond grains with each other. The use of a carbonate as the sintering binder also results in a compact with low fracture resistance. See, for example, Sumiya et al. U.S. Pat. No. 5,912,217.
Typical thermally stable polycrystalline diamond (TSP) compacts generally have a porosity wherein the pores have a high aspect ratio with a diameter of less than about 3, and generally less than about 1 micron, and a void volume of from approximately 10 to 2 percent. The pores are typically in the form of a reticulated network distributed throughout the TSP. It had been previously proposed to apply chemical vapor deposition procedures to deposit diamond in the pores of TSP compacts. See Pinneo U.S. Pat. No. 5,633,088. Chemical vapor deposition procedures generally do not produce satisfactory deposits where the average pore sizes are less than about 25 microns.
Bovenkerk at al. U.S. Pat. No. 4,224,380 proposed the formation of diamond or cubic boron nitride compacts wherein a mass of abrasive particles was sintered with a sintering aid under high temperature and pressure to form an abrasive compact in which the abrasive particles were self-bonded, and the sintering aid was infiltrated throughout the reticulated porosity of the compact. Removal of the infiltrant was accomplished by acid leaching. The resulting porous TSP compact resisted thermal degradation at high temperatures. As noted by Sumiya et al. U.S. Pat. No. 5,912,217, the resulting porous compacts are well known to have low strength and fracture resistance, and, as noted by Horton et al. U.S. Pat. No. 4,664,705, they are also known to oxidize rapidly at high temperatures. When silicon carbide is used as the sintering aid in the formation of polycrystalline diamond compacts the resulting compact exhibits excellent heat resistance but low strength and wear resistance because the binding of the diamond grains to one another is decreased. See Sumiya et al. U.S. Pat. No. 5,912,217. When silicon is used as the sintering aid in forming the polycrystalline diamond compact, the silicon reacts with the diamond and is converted to silicon carbide. The sintering process generally stalls before it is completed. The resulting compact has relatively poor wear characteristics. See Bunting et al. U.S. Pat. No. 5,127,923. Even when the sintering aid was leached from the resulting compact a certain amount of the sintering aid (0.05 to 3 volume percent) had typically remained in the compact. See Phaal et al. U.S. Pat. No. 4,534,773.
Horton et al. U.S. Pat. No. 4,664,705 proposed at least partially infiltrating, under heat and pressure (45-55 Kbars and above 1,000 degrees centigrade), previously formed self-bonded porous diamond compacts with a molten silicon containing alloy such as Ni—Si, Al—Si, or Cu—Si. Horton et al. suggested that because silicon alloys have coefficients of thermal expansion that are close to that of diamond, the at least partially infiltrated compact can withstand temperatures up to about 1200 degrees centigrade without cracking. Horton et al. also states the belief that the silicon in this system does not catalyze the conversion of diamond to graphite, and theorized that this contributed to the thermal stability of a silicon alloy infiltrated diamond compact. Horton et al. did not recognize that even the conversion of a small amount of diamond to silicon carbide or graphite seriously reduces the fracture toughness of the compact. From a consideration of the weight gain due to infiltrant (5-25 weight percent infiltrant), the known densities of the various materials, and the typical void volumes of polycrystalline diamond compacts (approximately 2 to 35 percent void volume), it appears that Horton et al. used very porous preforms in his examples and achieved considerably less than complete filling of the voids in the compact. It appears unlikely that Horton et al's. infiltrant penetrated uniformly throughout the preform. The typical resulting partially infiltrated compact would exhibit considerably less toughness than a substantially fully impregnated compact.
It is known that diamond adheres well to suicides such as Mo5Si3, Fe2Si, CoSi, Co2Si, Ni2Si and Fe3Si, and that a small amount of molybdenum carbide is formed when Mo5Si3 is used. See Casti U.S. Pat. No. 5,445,887. Casti did not recognize that Mo5Si3C, where the Mo5Si3 is saturated with carbon so that it does not attack the diamond, is chemically inert to diamond up to at least 2000 degrees centigrade.
A wide variety of pyrolyzable liquid polymeric materials had been proposed for use as ceramic precursors. Such materials include, for example, polysilazanes, polyureasilazanes, polythioureasilazanes, polycarbosilanes, polysilanes, polysiloxanes, siloxazanes, silsesquioxanes, silylated silicate resins, and the like. The inclusion of various organometallics in liquid ceramic precursors that yield metal silicates or silicides upon pyrolysis had been proposed. Typically, liquid ceramic precursors are cured to form a solid, which is then pyrolized to a ceramic form by heating at a rate of, for example, 200 degrees centigrade per hour to a final temperature of between approximately 300 and 900 degrees centigrade. The volume of the resulting ceramic, after pyrolysis, is typically from 20 to 80 percent by volume of the uncured liquid ceramic precursor. Conducting the pyrolysis operation in oxygen generally results primarily in the formation of silicon oxide related ceramics; in nitrogen, primarily the formation of silicon nitride related ceramics; and, in an inert atmosphere or vacuum, primarily the formation of silicon carbide or silicon oxycarbide ceramics. The inclusion of metals in the precursor results in the formation of ceramics that contain both silicon and the included metal.
The use of various liquid pre-ceramic infiltrants to infiltrate carbon-carbon preforms is known. The pore sizes in such preforms are generally at least approximately 10 microns or more in size.
Thermally stable polycrystalline diamond and cubic boron nitride preforms are known. The individual grains in such preforms can be doped with other materials, if desired. Diamond crystals have, for example, been doped with boron, and the like. Cubic boron nitride can also be doped with other materials, if desired. References herein to polycrystalline diamond or cubic boron nitride compacts, unless otherwise indicated, include compacts made from particles that contain such dopants. Dense superabrasive preforms that have a reticulated porosity of from about 2 to 10, preferably about 2 to 5 percent void volume, and an average pore size of from approximately 5 to 3,000 nanometers (approximately 0.005 to 3 microns) are also known. Such dense superabrasive preforms had generally been used for wear purposes rather than cutting applications. It had been generally assumed that fine structured diamond compacts acted like a large single crystal and were prone to crack propagation and catastrophic failure. It was also recognized that fine structured diamond-diamond compacts were difficult to infiltrate. See Cho et al. U.S. Pat. No. 5,151,107. Densification of diamond-diamond compacts containing less than about 3 volume percent void volume with average pore sizes of less than about 1,000 nanometers was generally considered to be impractical or impossible.
These and other difficulties of the prior art have been overcome according to the present invention.